The present invention relates to electrostatographic printing apparatus and in particular to an automatic toner dispensing apparatus for use with two component development apparatus.
In the process of electrostatographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is imagewise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a sheet of support material such as paper and permanently affixed thereto.
This process is well known and useful for light lens copying from an original and in printing applications from electronically generated or stored originals.
FIG. 1 is representative of a common configuration in electrostatographic printing apparatus wherein a two component magnetic brush development apparatus comprising a rotatable developer roll 10 rotating about fixed magnets 11 in the direction indicated by the arrow creating a magnetic field to attract two component developer to the surface of the developer roll is trimmed by a trim bar 12 to provide a uniform thickness of developer on the developer roll 10 to present this layer of developer to the surface of the photoreceptor 13 which has an electrostatic latent image on it's surface to attract the toner material from the developer roll surface to the surface of the photoreceptor in image configuration which is subsequently transferred electrostatically by transfer corotron 14 to a substrate 15 such as ordinary paper which is then detacked from the surface of the photoreceptor by detack corotron 18 and transported to the fuser (not shown) for fixing to the substrate. Any toner remaining on the surface of the photoreceptor is cleaned therefrom by a cleaner blade 19 and transported to a cleaner sump (not shown) by an auger 20. The developer, which comprises carrier particles such as steel shot and very much smaller thermoplastic resin toner particles is housed in the developer sump 21 with a paddle wheel 22 to mix the developer and present it to the developer roll to continually provide a supply of developer to the magnetic brush development roll 10. For additional details of this process, attention is directed to U.S. Pat. No. 2,874,063 and for further information about the process, in general, attention is directed to the book Electrophotography by R. M. Schaffert, Enlarged and Revised Edition; 1975 published in the U.S. by Halsted Press, a division of John Wiley & Sons, New York, both of which are hereby incorporated by reference. In this process, the individual toner particles are charged and attracted to the image on the photoreceptor thereby depleting the total developer supply in the developer sump of toner particles. Accordingly, it is necessary to replenish the toner in the developer to maintain a sufficiently high concentration of toner to ensure satisfactory image density in the final print. One previous approach to doing this is illustrated in FIG. I and also FIG. 4, wherein freshly supplied toner is supplied to one end of a toner dispensing tube 24 from a toner hopper 23. As previously discussed, the level of developer on a developer roll is controlled by a trim bar 12, which directs the excess developer back to the developer sump. As illustrated in FIGS. 1 and 4, while a majority of the developer trimmed by the trim bar flows to a baffle 25 with the majority falling back into the paddle wheel sump 21, the baffle 25 has a feature, such as a bridge 28 that allows a small amount of developer to be fed into the dispense tube 24 through an opening 27 at the end of the dispense tube where freshly supplied toner is introduced from the toner hopper. The dispense tube has an auger 29 in the tube which mixes the new toner with the feedback developer in the tube from the bridge 28. This facilitates the handling of toner since developer flows much more readily than toner. Physically, the carrier in the developer is about ten times the size of the toner particles, is much greater in mass and density, has additional different material characteristics which enable the feeding of a much larger volume of developer because of the feedback of a portion of the excess developer than of the toner alone. In this particular machine configuration the requirements for replenishing toner are so small that the toner only flow rate along the length of the tube would be very low and it would be very difficult to handle the small amounts of toner because of tolerances and clearances between the auger and the tube. Accordingly, the feedback flow of developer to the dispensing tube enables a more efficient transport of newly supplied toner and developer down the tube since the ratio of the carrier to toner is from 50 to 100 to one by mass. From the dispense tube the toner enriched developer is supplied to a mixing chamber 31 which has a wobble plate mixer 32 and is augered from one end of the dispensing tube down to the other end of the dispensing tube. The toner enriched developer is supplied to the mixing chamber through a slot 33 (see FIG. 2) in the bottom of the tube which gets increasingly wider down the length of the slot from the toner and developer input end to the opposite end of the dispensing tube. The wobble plate cross mixing device 32 is well known and comprises a rotary device with eliptical plates off axis to mix the toner and developer as the toner is dispensed through the mix in the mixing chamber 31. A dividing plate 35 is provided to limit the area in which mixed developer can flow to the development chamber 36.
While capable of performing satisfactorily under certain conditions it has been discovered that the feedback flow rate of developer from the developer roll trim bar to the dispensing tube will vary depending on the strength of the magnetic roll and the pick up of developer from the surface of the developer roll which varies from machine to machine due to differences in manufacturing tolerances. This variation in feedback flow rate into the dispensing tube causes variations in the dispensing uniformity out of the tube and into the mix chamber with the slotted tube configuration as may be observed with reference to FIG. 3, which illustrates this distribution of developer and toner over the width of the dispense tube at four different feedback flow rates. For high flow rate there is initially a very high dispensing intensity and at low flow rates there is essentially none. This nonuniform distribution results in poor developer uniformity in the mixing chamber and the developer housing resulting in uneven development uniformity and poor print quality.